Fate and Transport of Ambient Mercury and Applied Mercury Isotope in Terrestrial Upland Soils: Insights from the METAALICUS Watershed

The fate of mercury (Hg) deposited on forested upland soils depends on a wide array of biogeochemical and hydrological processes occurring in the soil landscape. In this study, Hg in soil, soilwater, and streamwater were measured across a forested upland subcatchment of the METAALICUS watershed in northwestern Ontario, Canada, where a stable Hg isotope (spike Hg) was applied to distinguish newly deposited Hg from Hg already resident in the watershed (ambient Hg). In total, we were able to account for 45% of the total mass of spike Hg applied to the subcatchment during the entire loading phase of the experiment, with approximately 22% of the total mass applied now residing in the top 15 cm of the mineral soil layer. Decreasing spike Hg/ambient Hg ratios with depth in the soil and soilwater suggest that spike Hg is less mobile than ambient Hg over shorter time scales. However, the transport of spike Hg into the mineral soil layer is enhanced in depressional areas where water table fluctuation is more extreme. While we expect that this pool of Hg is now effectively sequestered in the mineral horizon, future disturbance of the soil profile could remobilize this stored Hg in runoff

Optical Properties of Dissolved Organic Matter and Their Relation to Mercury Concentrations in Water and Biota Across a Remote Freshwater Drainage Basin

Dissolved organic matter (DOM) includes an array of carbon-based compounds that vary in size and structure and have complex interactions with mercury (Hg) cycling in aquatic systems. While many studies have examined the relationship between dissolved organic carbon concentrations ([DOC]) and methyl Hg bioaccumulation, few studies have considered the effects of DOM composition (e.g., protein-content, aromaticity). The goal of this study was to explore the relationships between total and methyl [Hg] in water, invertebrates, and fish and optically derived measures of DOM composition from 47 lake and river sites across a boreal watershed. Results showed higher aqueous total [Hg] in systems with more aromatic DOM and higher [DOC], potentially due to enhanced transport from upstream or riparian areas. Methyl [Hg] in biota were all positively related to the amount of microbial-based DOM and, in some cases, to the proportions of labile and protein-like DOM. These results suggest that increased Hg bioaccumulation is related to the availability of labile DOM, potentially due to enhanced Hg methylation. DOM composition explained 68% and 54% more variability in [Hg] in surface waters and large-bodied fish, respectively, than [DOC] alone. These results show that optical measures of DOM characteristics are a valuable tool for understanding DOM-Hg biogeochemistry

Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater

Changes in sulfate (SO42–) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO42– deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO42– export from a peatland-dominated catchment as a function of changing SO42– deposition in a long-term (1998–2011), whole-ecosystem, control-impact experiment. Annual SO42– deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO42– additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO42– addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO42– inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO42– deposition declines

Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater

Changes in sulfate (SO42–) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO42– deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO42– export from a peatland-dominated catchment as a function of changing SO42– deposition in a long-term (1998–2011), whole-ecosystem, control-impact experiment. Annual SO42– deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO42– additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO42– addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO42– inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO42– deposition declines

Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater

Changes in sulfate (SO42–) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO42– deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO42– export from a peatland-dominated catchment as a function of changing SO42– deposition in a long-term (1998–2011), whole-ecosystem, control-impact experiment. Annual SO42– deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO42– additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO42– addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO42– inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO42– deposition declines